Can be very hit and miss with ebay. Something like an ellaroo, or bear hug baby (Aus made RS that are very reasonably priced), or a Maya Wrap RS at least has a reputation of being a good quality sling that has been well tested and is an established brand.

If you do buy from ebay, you may be getting good quality, but just as likely that you may not. The quality of the fabric may not be great, or the shoulder stitching may not be secure (generally a secure shoulder will at least be triple stitched or reinforced in some other way), or the rings may not be quality rings made specifically for slings. I've seen slings on ebay that use wooden rings (dangerous), or seamed rings that can split at the join. A sling does go through a reasonable amount of stress when tightening and of course weight bearing on a regular basis, so it's one of those purchases that's worth spending a bit extra on to be sure of quality. There are quite a few makers out there who think - hey, it's a piece of fabric with a couple of rings at the end, how hard can it be, without knowing very much about babywearing at all.

One thing to check is how the sling is being worn in the promo photos (it's often difficult to see detail in the photo to guess at the quality). It's not uncommon to see slings that are of dubious quality being worn incorrectly - not tightened sufficiently, rings not in optimal position, baby seated poorly or knees below bum in a hip carry. If you go to a site like thebabywearer or google ring sling instruction video on you-tube, you'll get an idea of what a correctly worn ring sling looks like.